Many electrical devices, such as electronic electricity meters and induction or electronic type watthour meters for measuring electric power and energy usage, require current sensors for sensing the line current and producing an output signal related to the line current. These devices demand sensors which are accurate for a wide range of line currents.
For many years, induction type watthour meters have been used to measure energy consumption by individual electrical energy users, with consumption measured in kilowatt hours. Induction type watthour meters typically have separate voltage and current coils with a rotating disk driven by a combination of fluxes from the coils. Conventional induction watthour meters include a current sensing circuit in which the conductor carrying the line current is wrapped about in an iron core to form a current coil. A magnetic flux is created in the iron core due to the current in the conductor. The magnetic flux in the iron core, in combination with the magnetic flux from a similar voltage coil, rotates a disk at a rate related to the customer's electrical energy consumption rate.
Current transformers are used in conjunction with the above-described current sensing circuits to scale down relatively large line currents, i.e., greater than about 320 amperes ("A"), since the number of turns about the iron core required to properly scale such relatively large line currents would become prohibitively large. The current transformer is disposed between the line conductor and the current sensing circuit. While such current transformers are generally rated to have a nominal current in their secondary winding of 5 A, approximately 20 A of secondary current may usually be drawn without exceeding the thermal rating of the transformer. Thus, even with the introduction of a current transformer between the conductor and the current sensing circuit, induction type watthour meters must still be capable of measuring relatively large currents, such as 20 A.
Such relativity large currents, however, cannot be accurately sensed by electronic electricity meters, such as electronic watthour meters or other electronic metering devices which typically utilize electronic or integrated circuits for measuring the current and voltage usage of individual electrical energy customers. The integrated circuits are generally application specific integrated circuit ("BASIC") which are designed to accept and measure small signal levels, such as typically less than 2 mA and less than 5 volts. The current sensors in electronic watthour meters, therefore, must have a large line transformation ratio to scale the relatively large line currents to the relatively small input levels accepted by the sensor's integrated circuits.
In order to produce such relatively small output signals, typical current transformers would become prohibitively large and expensive. This increased size and expense is due, in part, to the requirement that the ampere turns of the primary and secondary windings must be equal for proper operation. The number of windings must also be selected such that the maximum input line current is scaled to a value less than the 2 mA current limit for the integrated circuit. Since line currents typically vary from 0.5 A to 320 A, the transformation ratio of typical current transformers would need to be approximately 160,000:1 to convert 320 A of line current to a scale output current of 2 mA. A transformation ratio of 160,000:1, however, would require a relatively large number of windings and a prohibitively large and expensive current transformer.
In addition to being prohibitively large and expensive, the magnetic cores of typical current transformers saturate if the alternating current ("AC") flowing in a line conductor is superimposed upon a direct current ("DC"). The direct current, and thus the saturation of the magnetic core, is generally due to half-wave rectification of AC signals by various electrical devices connected to the line conductor or by persons intentionally superimposing DC components upon the line conductor to commit meter fraud by preventing proper current sensing and electrical energy consumption measurement.
Furthermore, typical current transformers produce an external magnetic field which may affect adjacent electrical devices, such as other current transformers in a polyphase watthour meter. The current transformation in typical current transformers may also be adversely affected by incident magnetic fields from external sources such as adjacent current sensors employed in polyphase watthour meters.
An alternative current sensor to such typical current transformers is provided in U.S. Pat. No. 4,182,982 to Wolf et al. which issued January 1980 and U.S. Pat. No. 4,492,919 to Milkovic which issued January 1985 (hereinafter the '982 and '919 patents, respectively). The '982 and '919 patents disclose the division of a line conductor into one or more main shunt paths and a parallel auxiliary shunt path having different cross-sectional areas. The current divides between these two shunt paths substantially in proportion to the cross-sectional areas of the two paths. The auxiliary shunt path passes through the bore of a toroidal magnetic core. A current transformer is formed by the combination of the toroidal magnetic coil, the auxiliary shunt path which forms a one turn primary winding, and a winding of many turns wound about the toroidal magnetic core which forms a secondary winding.
The cross-sectional areas of the primary shunt path and the auxiliary shunt path, however, may become prohibitively large and small, respectively, in order to properly scale the line current over the large range of potential line currents. Furthermore, the parallel primary and auxiliary shunt paths are affected not only by magnetic fields produced by external sources, but also by magnetic fields produced by the current in the other shunt paths. For example, the current in the primary shunt path is affected by the magnetic field produced by the current in the auxiliary shunt path. In addition, the magnetic coupling between the parallel conductors in the current divider produces a mutual inductance between the two parallel conductors. This inductance transforms the relatively simple resistance divider to a complex impedance divider with the phase shift of the current in the main shunt path and parallel auxiliary path dependant on the inductance. Since adequate metering accuracy demands that both the magnitude and the phase angle of the scaled output signal of the current sensor accurately reflect the magnitude and the phase angle of the line current, such phase shifts in the parallel shunt paths, which, in turn, are reflected in the output signal of the current sensor impair metering accuracy.
A coaxial current sensor is disclosed in U.S. Pat. No. 5,066,904 to Bullock which issued on Nov. 19, 1991 and is assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. The coaxial current sensor divides the current in the line conductor between two coaxially-arranged conductors. The center coaxial conductor is directed through the bore of a magnetic toroidal core and induces magnetomotive force in the toroidal core.
Sense and feedback secondary windings are also wound about the toroidal core. A voltage is induced in the sense winding due to and in proportion to the time-rate of change of the magnetomotive force in the toroidal core. An amplifier responsive to the voltage induced in the sense secondary winding provides a control or compensation signal to the feedback secondary winding. The current in the feedback winding due to the control signal produces a magnetomotive force in the toroidal core, substantially equal in magnitude and opposite in polarity to the magnetomotive force induced by the current in the center coaxial conductor. The resultant net AC magnetomotive force in the magnetic toroidal core is approximately zero in a steady-state condition. Thus, the likelihood of saturation of the core is significantly diminished. Further, any current in the magnetic toroidal core is primarily due to changes in the input current is induced in relation to the current in the center coaxial conductor. The feedback second secondary winding also produces the output current which is proportional to the current in the center coaxial conductor.
A differential current sensor is disclosed in U.S. patent application Ser. No. 08/043,903 filed Apr. 7, 1993 and assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. The differential current sensor divides an input line current into first and second portions having a predetermined ratio therebetween. The difference in current between the first and second portions is thereafter sensed, such as by a toroidal winding which is magnetically coupled to the first and second portions of the input current. Based upon the sensed current differential between the first and second portions, an output current is produced related to the current differential and, thus, related to the input current.
In particular, the input current divides into first and second portions and is conducted by first and second conductors, respectively, which extend through a bore of the toroidal winding. The first portion of the input current flows in a first direction through the bore of the toroidal winding while the second portion of the input current is in a second direction, opposite the first direction, through the bore of the toroidal winding. Thus, the current differential between the first and second portions induces a voltage in the toroidal winding. The differential current sensor preferably includes means for magnetically sensing the voltage induced in the toroidal winding and producing an output current in response thereto. The means for magnetically sensing the current differential is typically sense and feedback secondary windings wound about the toroidal magnetic core as described above.
While the coaxial and differential current sensors represent great advances in the art of sensing and scaling line currents, particularly for use in electricity meters, such as electronic watthour meters, further improvements are desirable. In particular, the coaxial conductors of the coaxial current sensor magnetically couple due to the magnetic field produced by the current in each conductor and the resulting self-inductance in the other conductor. This mutual inductance introduces a phase shift or error in the resulting scaled output current.
In addition, since the ratio at which the current divides between the coaxial conductors depends upon the cross-sectional areas of the conductors, the ratio of current division may be adjusted by trimming the resistances of the conductors, such as by drilling a portion of the material from a first end of the conductors. This removal or drilling of the conductors may be difficult due to the alignment and size of the conductors. Furthermore, both the coaxial and the differential current sensors are of relatively complex design and, accordingly, may be expensive to fabricate.
Still further, the bore of the toroidal core of the differential current sensor is substantially perpendicular to the first and second portions of current conducted by the current sensor outside of the bore. Accordingly, the magnetic flux induced by the current conducted by the current sensor outside of the bore of the toroidal core magnetically couples to the sense and feedback secondary windings so as to introduce a phase shift in the scaled output current. Furthermore, both the coaxial and the differential current sensors are of relatively complex design and, accordingly, may be expensive to fabricate.
Thus, while it would be desirable to have a current sensor for producing an output current having a magnitude and phase angle related to an input current, particularly for use in sensing line currents in electricity meters, such as electronic watthour meters, current sensors still suffer from a number of deficiencies, including complex and costly designs and undesirable magnetic coupling which results in phase shifts in the output signal. In particular, it would be desirable to have a current sensor having a relatively simple design which produces an output signal having a magnitude of approximately 2-3 mA which is related to the magnitude and phase of the input current for use with electricity meters which utilize electronic or integrated circuits for measuring a customer's electrical energy consumption.